Choose an already made layout (Like: KeyboardLayout_ES.h) and copy the contents to a new file named "KeyboardLayout_*.h" where "*" is your country code in caps E.g: "KeyboardLayout_ES.h"
Now that you have your file, start by modifying extern const uint8_t KeyboardLayout_es_ES[128]
For this I'll just copy the already made tutorial in https://github.com/arduino-libraries/Keyboard/blob/master/src/KeyboardLayout.h
== Creating your own layout ==
In order to create your own layout file, copy an existing layout that